1. Field of the Invention
The present invention relates to a receiver, receiving method, program and receiving system, and more particularly, to a receiver, receiving method, program and receiving system whose circuit scale can be reduced.
2. Description of the Related Art
Recent years have seen the use of a modulation scheme called orthogonal frequency division multiplexing (OFDM) for transmitting digital signals. This OFDM is a digital modulation scheme that uses a number of subcarriers orthogonal to each other in a transmission band. Data is assigned to the amplitude and phase of each of the subcarriers. Digital modulation is accomplished by phase shift keying (PSK) or quadrature amplitude modulation (QAM).
The OFDM scheme is often used for terrestrial digital broadcasting that is severely affected by multipath interference. Among terrestrial digital broadcasting standards using OFDM are DVB-T (Digital Video Broadcasting-Terrestrial) and ISDB-T (Integrated Services Digital Broadcasting-Terrestrial).
Incidentally, DVB-T2 (second generation European terrestrial digital broadcasting standard) is on its way to being developed as a terrestrial digital broadcasting standard using OFDM.
It should be noted that DVB-T2 is described in the so-called Blue Book (DVB BlueBook A122) (“Frame structure channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2)”, DVB Document A122 June 2008).
In DVB-T2 (the Blue Book thereof), a frame called a T2 frame is defined so that data is transmitted in units of a T2 frame. A T2 frame contains two preamble signals called P1 and P2. These preamble signals contain information required for processes such as demodulation of an OFDM signal.
In DVB-T2, on the other hand, a scheme called M-PLP (Multiple PLP (Physical Layer Pipes)) is used. With M-PLP, data is transmitted using two types of packet sequences. One of them is a plurality of packet sequences (data packet sequences) called data PLPs. The other is a packet sequence (common packet sequence) called a common PLP. A data PLP includes a packet remaining after a packet (information) common to all of a plurality of original transport streams (each of which will be hereinafter referred to as a TS) has been extracted. A common packet sequence includes a common packet. In other words, a common packet includes a packet common to a plurality of TSs, whereas a data PLP includes a packet specific to one of the plurality of TSs. On the receiving side, an original TS is reconstructed from common and data PLPs.
That is, a data PLP is an individual piece of service information, and a common PLP is a common piece of information extracted from two or more data PLPs. Therefore, the relationship data PLP count≧2× common PLP count≧0 holds. As a result, there is a multiple-to-one relationship between data PLPs and common PLP. For a given common PLP, there are two or more data PLPs. For a given data PLP, there is a common PLP.
FIG. 1 is a diagram illustrating a T2 frame configuration.
When the transmitting side transmits a T2 frame which includes a common PLP, data PLP#1, data PLP#2 and so on with the common PLP being a piece of information common to a plurality of data PLPs, the receiving side proceeds as follows when receiving this T2 frame. That is, if the data PLP#2 is specified, the receiving side selects the data PLP#2 and the common PLP accompanying the data PLP#2 so that the original information can decoded from these selected PLPs.
At the time of decoding, these two PLPs, i.e., the common PLP and data PLP, have to be decoded at the same time. Further, DVB-T2 performs time interleaving to enhance instantaneous noise immunity in the time direction. Time interleaving randomizes data in the time direction.
A time deinterleaver used in DVB-T2 can start its output while at the same time time-deinterleaving the time-interleaved PLP when the input of a predetermined unit of data to be processed is complete. In this time deinterleaver, therefore, the input and output timings are not in a one-to-one correspondence with each other.
An error correction section is provided at the succeeding stage of the time deinterleaver. The error correction section performs error correction on the data that has been sorted by the time deinterleaver.
If two time deinterleavers are provided, one for the common PLP and another for the data PLP, two configurations are possible as illustrated in FIGS. 2A and 2B, one in which two error correction sections are provided, one for each PLP (FIG. 2A) and another in which one error correction section is provided (FIG. 2B) so that it is shared by the two PLPs. Commonly, the configuration shown in FIG. 2B is used in which a single error correction section is shared by the two PLPs for reduced circuit scale and reduced power consumption. Therefore, a description will be given below of a case in which a single error correction section is shared by the two PLPs.
In FIG. 2B, each of the time deinterleavers performs time deinterleaving in units of information to be processed called a TI-block (Time Interleaving block). These time deinterleavers output data to the common error correction section in units of information to be output called an FEC block. The relationship between these units of information to be processed is as shown in FIGS. 3A and 3B.
As illustrated in FIGS. 3A and 3B, one TI-block corresponds to a plurality of FEC blocks. However, when the number of TI-blocks in an interleaving frame is denoted by NTI, this NTI may vary. That is, when NTI=1 as illustrated in FIG. 3A, the interleaving frame is equal to the TI-block. On the other hand, when NTI=3 (NTI>1) as illustrated in FIG. 3B, the interleaving frame contains three TI-blocks. Therefore, the interleaving frame is not equal to the TI-block.
In the case of NTI=1 shown in FIG. 3A, each of the time deinterleavers sorts data in the input PLP (common or data PLP) in units of a TI-block as illustrated in FIG. 4. When NTI=1, the interleaving frame is equal to the TI-block. Each of the time deinterleavers receives a PLP in units of a TI-block. When the input of the TI-block data is complete, each of the time deinterleavers begins its output while at the same time time-deinterleaving the time-interleaved PLP so that the sorted data is output to the common error correction section.
On the other hand, when NTI=3 (NTI>1) as illustrated in FIG. 3B, the interleaving frame is not equal to the TI-block. Therefore, data is sorted in units of each of three TI-blocks or TI-block0, TI-block1 and TI-block2, for a single interleaving frame as illustrated in FIG. 5. However, a memory area available with the time deinterleaver is large enough only for a single TI-block. As a result, as soon as the input of the first TI-block or TI-block0 is complete, the time deinterleaver has to initiate its output. Otherwise, the memory storing the TI-block0 is overwritten by the next TI-block or TI-block1.
In particular, if, while one time deinterleaver outputs a PLP, the other time deinterleaver completes its input of the first TI-block or TI-block0 among the TI-blocks where NTI>1, the other time deinterleaver may not initiate its output because the one time deinterleaver is outputting its data to the common error correction section. This causes the stored TI-block0 to be overwritten, resulting in data loss.